What tells us it is extraordinarily cold outside?

It was colder than 50 below zero. The man knew this because his spit turned to ice before it hit the ground.

And again, in the air, before they could fall to the snow, the
drops of water became ice that broke with a noise. He knew that at 50
below zero water from the mouth made a noise when it hit the snow.
But this had done that in the air. Undoubtedly it was colder than 50
below. But exactly how much colder he did not know. But the temperature did not matter.